Sensing: Jacobson, IR, UV

Squamates flick a bifid tongue to sample molecular cues from substrate and air; the tongue tips deposit molecules onto the paired openings of the vomeronasal organ (VNO) in the hard palate. Each tongue fork contacts one VNO separately, giving a stereo gradient, so the snake or lizard performs molecular tropotaxis (Schwenk 1995).

The VNO sensory neurons express V1R and V2R G-protein-coupled receptors (Young 2005) and signal through a TRPC2-dependent cascade. Different from the main olfactory system, the VNO is specialised for pheromones and prey-trail molecules (lipids, steroids). Martinez-Marcos 2009 describes the conserved archicortical projection pattern.

Crotalines (rattlesnakes, copperheads, fer-de-lances) and boids carry paired loreal pit organs that image infrared radiation at ~8–12 µm. The organ is a pinhole camera whose pit membrane carries a dense field of thermosensitive nerve endings. Gracheva 2010 (Nature) identified TRPA1 as the molecular sensor: cloned pit-viper TRPA1 in heterologous cells responds to temperature jumps as small as 0.003 K, consistent with the neural threshold.

Geometry gives angular resolution ~d/L where d is the aperture (∼1 mm) and L the depth (∼3 mm): roughly 20° per sensor, refined by neural cross-correlation across hundreds of terminals to ~5° effective resolution. The tectum-superior colliculus homolog integrates visual and IR signals into a unified prey map (Newman & Hartline 1981).

\[ \Delta W = \sigma(T_{prey}^4 - T_{amb}^4),\quad \Delta T_{mem} = \frac{\Delta W\cdot A\cdot\tau}{m\cdot c} \]

With τ ∼ 30 ms, m ∼ 1 µg membrane mass, c ∼ 3.5 kJ kg^-1 K^-1, a 36 °C mouse against a 20 °C background produces ~10 mK membrane heating — well above the TRPA1 threshold.

Many lizards, all Sphenodon, and some turtles retain a functional parietal eye embedded in the skull roof — a small photoreceptor cluster with a lens and a retina but no image formation. Its function is a scalar UV dosimeter: Tosini 1996 showed the parietal eye entrains circadian rhythms and modulates thermoregulatory setpoint; Solessio 1998 demonstrated chromatic opponent photoreceptors signalling blue/green ratio.

The parietal eye connects to the pineal gland; melatonin output encodes photoperiod, feeding into seasonal reproductive physiology (M6). In an evolutionary sense, the parietal eye is a retained third eye; mammals and birds have converted the homologous structure into a purely endocrine pineal.

Snake eyes are notable for several derived features. The ancestral snake appears to have been a nocturnal burrower; descendants have restored diurnal vision by different routes (Simoes 2015). Most snakes lack a classical cone-rod duplex; instead photoreceptors have evolved a mosaic of “transmuted” cell types. The spectacle (brille) is a clear epidermal scale covering each eye — shed with the rest of the skin during ecdysis, producing the characteristic opaque-eye phase. Boids and pythons add IR from the loreal pit to their visual feed, and some highly visual arboreal snakes (Ahaetulla) have forward-facing binocular fields achieved by grooved snouts that clear the visual axis.